The ever-increasing demand for broadband communication systems has led to optical transmission systems based on optical waveguides such as fiber optics and optical processing elements for use in these systems. To decrease the cost of such systems, large-scale integration of the optical devices is needed. Large-scale integration also provides decreased size and increased reliability.
One optical device that is used in such systems is an optical isolator. In optical communication systems, optical signals pass through interfaces that generate reflected signals that propagate back to the signal source. Optical isolators are used to block these reflected signals from reaching the source. Ideally, optical isolators transmit the optical signal in the forward direction and block the reflected light signal that is traveling in the reverse direction.
Optical isolators based on birefringent crystals, polarizers, and Faraday rotators are well known in the optical signal arts. Isolators based on Faraday rotators operate by rotating the polarization of the light in a direction that depends on the direction of travel of the light relative to the direction of an applied magnetic field. The amount of rotation θ experienced by the input plane of polarization is proportional to the length, L, of the Faraday medium, the magneto-optic or “Verdet” coefficient, V, of the medium and the strength of the applied magnetic field, H.θ=VHLThe simplest devices operate on linearly polarized light and consist of two polarization filters and a Faraday rotator that is located between the polarization filters. Light entering the device in the “pass direction” has a polarization that is aligned to pass through the first polarization filter. The Faraday rotator causes the direction of polarization of this light to be rotated by the Faraday rotator such that it will pass through the second polarization filter without being blocked. The polarization of any light traveling in the reverse direction that passes through the second polarization is rotated by the Faraday rotator to a direction that is blocked by the first polarization filter.
The above described isolator assumes that the light is linearly polarized in a direction that matches the pass direction of the first polarization filter. If the incident light is not linearly polarized, the light must be first split into two linearly polarized components and two isolators are used, one per component. If the light leaving the isolator is to have the same polarization as that entering the isolator, quarter waveplates must be included to rotate the light to the original polarization direction.
Such devices are relatively large and expensive, and hence, poorly suited for use in large-scale optical signal processing systems. In particular, the thickness of the Faraday rotator cannot be reduced, since the amount of rotation per unit length of material in the rotator depends on the physical properties of the materials used. Hence, there is a minimum thickness for the Faraday rotator and any quarter waveplates. These devices are typically of the order of 1 mm thick, which is large by integrated circuit standards. In addition, integration of these optical isolators with waveguides and other processing optical elements that are formed on a common substrate is difficult with these designs.